Tamm began her acting career on the stage with the Birmingham Repertory Company in 1971, moved to London in 1972 and appeared in the musical Mother Earth. Before her association with Doctor Who, Tamm acted in several motion pictures, most notably Sigi in The Odessa File (1974) and The Likely Lads (1976).

Tamm was not initially interested in playing a companion to the Doctor. She believed the role was merely that of the "damsel in distress." She changed her mind when assured by the producers and her agent that Romana would be different. While Romana was supposed to be a Time Lady, a member of the Doctor's own people and therefore as capable as the Doctor, the character eventually took on the characteristics that Tamm was concerned about; as a result of this, she left the programme after only one season.[2]

Other reasons for her departure were reported in media and in fandom, but this is the only reason mentioned in the recent Key to Time DVD box set. The role of Romana was assumed by Lalla Ward. On the 2007 Special Edition DVD release of the Key to Time, Tamm states she was willing to film a regeneration sequence, but was not asked to do so.

Since then, Tamm acted in film and television, playing the characters of Penny Crosbie in the soap opera Brookside from 1993 to 1995, Yvonne Edwards in the BBC drama Paradise Heights (2002) and guest roles in many television programmes. In one of her films, the 1987 release Three Kinds of Heat, her character has the dubious distinction of being killed off by a villain portrayed by future Doctor Sylvester McCoy.

Tamm returned to the role of Romana in the second series of Gallifrey audio plays produced by Big Finish Productions. In 2007 she added "interviewer" to her CV when she hosted and conducted interviews for a brief documentary entitled "Stones Free" for the DVD release ofThe Key to Time, in which she talked to historians about one of the locations for the serial, The Stones of Blood. The 2|entertain documentary There's Something About Mary... saw Tamm recount her time on the show and the means by which she acquired and later left the role.

She performed in the Big Finish production The Stealers from Saiph (released June 2009), again playing Romana, as well as reading the voice of the Fourth Doctor.

She was married with one daughter and a grandson. On 14 September 2009, Tamm published her autobiography, First Generation — a reference to the fact she was in the first generation of her family born in the UK, as her parents hailed from the then-Soviet Union.[3] She also recorded an audio book version. First Generation was publicised as "volume 1" of her autobiography, suggesting a follow-up book in the future.

In early 2010, 2|entertain announced a new series of DVD featurettes entitled Tomorrow's Times would be featured on upcoming DVD releases. The first chapter of this series was presented by Mary Tamm.[4]

In 2011 and 2012, Tamm recorded a series of Big Finish audios as Romana alongside Tom Baker. These were released from January to July 2013.

On 24 July 2012, Tamm died at the age of 62. The BBC confirmed that Tamm died in hospital following a battle against cancer. Tom Baker said in a statement: "She was a darling companion and wonderfully witty and kind. I'm so sorry to hear of her death." There was some initial confusion in the media regarding the actual date of her death, with the Radio Times later confirming that she died the morning on 24 July.[5]